Lateral Distribution of a Pyrene-Labeled Phosphatidylcholine in Phosphatidylcholine Bilayers: Fluorescence Phase and Modulation Study

The lateral distribution of 1-palmitoy1-2-[10-(1-pyrenyl)decanoyl]phosphatidylcholine (PyrPC) was studied in small unilamellar vesicles of 1,2-dipalmitoyl-, 1,2-dimyristoyl-, and l-palmitoyl-2-oleoylphosphatidylcholine (DPPC, DMPC, and POPC, respectively) under anaerobic conditions. The DPPC and DMPC experiments were carried out over temperature ranges above and below the matrix phospholipid phase transition temperature (T_m). The excimer to monomer fluorescence intensity ratio (E/M) was determined as a function of temperature for the three PyrPC/lipid mixtures. Phase and modulation data were used to determine the temperature dependence of pyrene fluorescence rate parameters in gel and in liquid-crystalline bilayers. These parameters were then used to (1) provide information about excited-state fluorescence in phospholipid bilayers, (2) calculate the concentration of the probe within liquid-crystalline and gel domains in the phase transition region of PyrPC in DPPC, and (3) simulate E/M vs. temperature curves for three systems whose phase diagrams are different. From the simulated curves we could determine the relationship between the shape of the three simulated E/M vs. temperature curves and the lateral distribution of the probe. This information was then used to interpret the three experimentally derived E/M vs. temperature curves. Our results indicate that PyrPC is randomly distributed in pure gel and fluid phosphatidylcholine bilayers. In the phase transition region, PyrPC molecules preferentially distribute in fluid phases in DPPC but distribute almost equally between fluid and gel phases in DMPC. There is, however, no gel-phase immiscibility in these systems.